Adrenoreceptors (ARs) are membrane proteins belonging to the superfamily of G-protein-coupled receptors (Hoffman et al., “Radioligand Binding Studies of Adrenergic Receptors: New Insights into Molecular and Physiological Regulation,” Annu. Rev. Pharmacol. Toxicol 20:581-608 (1980); Gerhardt et al., “Multiple Gi Protein Subtypes Regulate a Single Effector Mechanism,” Mol. Pharmacol. 40:707-711 (1991); Eason et al., “Simultaneous Coupling of α2-Adrenergic Receptors to Two G-proteins with Opposing Effects: Subtype-selective Coupling of α2C10, α2C4, and α2C2 Adrenergic Receptors to Gi and Gs,” J. Biol. Chem. 267:15795-15801 (1992)). With the aid of pharmacological and molecular biological techniques, the α-adrenoreceptor subtypes α1 and α2 were determined. Detailed studies have since shown that these initial subtypes are further divided into α1A, α1B, α1D, α2A, α2B, α2C, and α2D subtypes, depending on species and tissues (Bylund, “Subtypes of α1- and α2-Adrenergic Receptors,” FASEB J. 6:832-839 (1992); Bylund et al., “International Union of Pharmacology Nomenclature of Adrenoceptors,” Pharmacol. Rev. 46:121-146 (1994); Hieble et al., “α- and β-Adrenoceptors: From the Gene to the Clinic. 1. Molecular Biology and Adrenoceptor Subclassification,” J. Med. Chem. 38:3415-3444 (1995); Hieble et al., “Subclassification and Nomenclature of α1 and α2-Adrenoceptors,” Prog. Drug Res. 47:81-130 (1996); Hieble, et al., “Functional Subclassification of α2-Adrenoceptors,” Pharmacol. Commun. 6:91-97 (1995)).
This knowledge has led to a search for selective agonists and antagonists for each subtype. Although there are a number of α2-AR antagonists (Ruffolo et al., “α- and β-Adrenoceptors: From the Gene to the Clinic. 2. Structure-Activity Relationships and Therapeutic Applications,” J. Med. Chem. 38, 3681-3716 (1995); Clark et al., “Pharmacology and Structure-Activity Relationships of α2-Adrenoceptor Antagonists,” Prog. Med. Chem. 23:1-39 (1986)), only a small set of compounds have shown even a degree of selectivity among the three subtypes of α2-AR. However, these compounds suffer from either low subtype selectivity or binding to receptor sites outside the α2-AR subfamily (Ruffolo et al., “α- and β-Adrenoceptors: From the Gene to the Clinic. 2. Structure-Activity Relationships and Therapeutic Applications,” J. Med. Chem. 38, 3681-3716 (1995); Okumura et al., “The Selectivity of Newly Synthesized Ergot Derivatives to α1- and α2-Adrenoceptors, D1- and D2-Dopaminergic Receptors, Muscarinic Acetylcholinoceptors and β-Adrenoceptors,” Gen. Pharmacol. 19:463-466 (1988); Beeley et al., “Synthesis of a Selective α2A Adrenoceptor Antagonist, BRL 48962, and its Characterization at Cloned Human α-Adrenoceptors,” Bioorg. Med. Chem. 3:1693-1698 (1995); Blaxall et al., “Characterization of the α2C Adrenergic Receptor Subtype in the Opossum Kidney and in the OK Cell Line,” J. Pharmacol. Exp. Ther. 259:323-329 (1991); Bylund et al., “Pharmacological Characteristics of α2-Adrenergic Receptors: Comparison of Pharmacologically Defined Subtypes with Subtypes Identified by Molecular Cloning,” Mol. Pharmacol. 42:1-5 (1992)).
Efforts made towards understanding the biological significance of each of the α2-adrenergic receptor subtypes (Bylund et al., Adrenoceptors, in The IUPHAR Compendium of Receptor Characterization and Classification, 1st ed (1998), pp 58-74, IUPHAR Media Company, Burlington Press, Cambridge, England) have resulted only in marginal success due to the lack of subtype-selective ligands. The significance of functional groups in imidazoline compounds affecting selectivity and affinity in the α2-AR system were detailed recently by Pigini et al. (Pigini et al., “Imidazoline Receptors: Qualitative Structure-Activity Relationships and Discovery of Tracizoline and Benazoline, Two Ligands with High Affinity and Unprecedented Selectivity,” Bioorg. Med. Chem. 5:833-841 (1997); Pigini et al., “Structure-Activity Relationship at α-Adrenergic Receptors Within a Series of Imidazoline Analogues of Cirazoline,” Bioorg. Med. Chem. 8:883-888 (2000); Gentili et al., “α2-Adrenoreceptors Profile Modulation and High Antinociceptive Activity of (S)-(−)-2-[1-(biphenyl-2-yloxy)ethyl]-4,5-dihdryo-1H-Imidazole,” J. Med. Chem. 45:32-40 (2002); Gentili et al., “Imidazoline Binding Sites (IBS) Profile Modulation: Key Role of the Bridge in Determining I1-IBS or I2-IBS Selectivity Within a Series of 2-Phenoxymethylimidazoline Analogues,” J. Med. Chem. 46:2169-2176 (2003); Gentili et al., “α2-Adrenoreceptors Profile Modulation. 2. Biphenyline Analogues as Tools for Selective Activation of the α2C-subtype,” J. Med. Chem. 47:6160-6173 (2004)).
This endeavor has been greatly assisted by genetic manipulation using mice with deletions, mutations, or overexpression of specific α2-AR subtypes. The role of the α2C-, in addition to the α2A-AR, in the feedback control of neurotransmitter release is a finding from one such study (Hein et al., “Two Functionally Distinct α2-Adrenergic Receptors Regulate Sympathetic Transmission,” Nature 402:181-184 (1999)). Contribution of the α2C-ARs to α2-AR opioid synergy induced by certain agonists such as moxonidine is another finding (Fairbanks et al., “α2C-Adrenergic Receptors Mediate Spinal Analgesia and Adrenergic-Opioid Synergy,” J. Pharmacol. Exp. Ther. 300:282-290 (2002)). Together, these findings suggest that the α2C-AR may represent a better therapeutic target for analgesic therapy than the α2A-AR, since this subtype would also lead to fewer sedative effects.
In the central nervous system, the α2C-ARs appear to have a distinct inhibitory role in various CNS-mediated behavioral and physiological responses including startle reactivity, aggressive behavior, and amphetamine-induced locomotor hyperactivity (Scheinin et al., “Evaluation of the α2C-adrenoceptor as a Neuopsychiatric Drug Target: Studies in Transgenic Mouse Models,” Life Sci. 68:2277-2285 (2001)). Increased α2C-AR activity may lead to, or result from, a constitutively stressful state thereby causing depression. α2C-AR subtype-selective drugs, therefore, may be useful in a variety of neuropsychiatric disorders (Scheinin et al., “Evaluation of the α2C-adrenoceptor as a Neuopsychiatric Drug Target: Studies in Transgenic Mouse Models,” Life Sci. 68:2277-2285 (2001)).
Beside these findings derived from gene-targeted mice, a recent study (Chotani et al., “Silent α2C-adrenergic Receptors Enable Cold-induced Vasoconstriction in Cutaneous Arteries,” Am. J. Physiol. Heart Circ. Physiol. 278:1075-1083 (2000)) has identified yet another therapeutic use for an α2C-AR antagonist. The study showed that at lower temperatures the α2C-ARs are principally responsible for mediating the cold-induced augmented vasoconstrictor response. This subtype, however, did not contribute to α2-AR dependent vasoconstriction at 37° C. A selective inhibition of the α2C-ARs in microvessels has, thus, been proposed to provide an effective treatment for cold-induced cutaneous arterial blood vessel constriction as observed in Raynaud's phenomenon. The importance of α2C-AR antagonists in treating Raynaud's disease was illustrated by Flavahan et al. (U.S. Pat. No. 6,444,681 to Flavahan et al.; Chem. Abstr. 137:103 (2002)). The most common probes used in these studies are agonists such as clonidine, and antagonists such as yohimbine and yohimbine like compounds viz. rauwolscine, corynanthine.
FIG. 1 illustrates the structure of several prior art compounds, including yohimbine 1, which is known to be a potent α2-AR antagonist, and has been used extensively as a pharmacological probe for studying the α2-AR (Goldberg et al., “Yohimbine: A Pharmacological Probe for Study of the α2-Adrenoreceptor,” Pharmacol. Rev. 35:143-180 (1983)). To improve subtype selectivity, the bivalent ligand approach was introduced recently based on the concept that a bivalent ligand should first undergo univalent binding, followed by the binding of the second pharmacophore to a recognition site on a neighboring receptor (Portoghese, Portoghese, 2000 Alfred Burger Award Address in Medicinal Chemistry, “From Models to Molecules: Opioid Receptor Dimers, Bivalent Ligands, and Selective Opioid Receptor Probes,” J. Med. Chem. 44: 2259-2269 (2001)). Using this approach, several yohimbine dimers were prepared with methylene and methylene-diglycine spacer linkages. It was discovered that such compounds with spacers of n=3(2) and n=24(3) showed the highest potency and selectivity for the α2C-AR in receptor binding studies and in functional studies measuring cAMP changes using a cell-based luciferase reporter gene assay (Zheng et al., “Yohimbine Dimers Exhibiting Binding Selectivities for Human α2a- Versus α2b-Adrenergic Receptors,” Bioorg. Med. Chem. Lett. 10:627-630 (2000); Lalchandani et al., “Yohimbine Dimers Exhibiting Selectivity for the Human α2C-AR Subtype,” J. Pharmacol. Exp. Ther. 303, 979 (2002); U.S. Pat. No. 6,638,943 to Miller et al.; Chem. Abstr. 137:794 (2002)). Interestingly, none of the dimer analogs surpassed the affinity of yohimbine.
Despite the prior advances concerning pharmaceutical selectivity of α2-adrenoceptor subtypes, it would be desirable to identify other compounds with selectivity for particular of α2-adrenoceptor subtype such as the α2C-AR, and preferably compounds that exhibit both high affinity and receptor subtype selectivity.
The present invention is directed to overcoming these and other deficiencies in the art.